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2021 Vol. 26, No. 3
Published: 2021-06-15
Engineering and Technology
Theoretical Research
Theoretical Research
189
Diffusion and atomic mobility of high temperature BCC phase in Ti-Al-Cr system
WU Chenjian, BAI Weimin, GAO Ning, LIU Libin, ZHANG Ligang
The diffusion study of titanium and its alloys can benefit to predict the phase constitution and microstructure evolution of titanium alloys during heat treatment. In this work, the diffusion behavior in BCC phase of Ti-Al-Cr ternary system was studied through diffusion couples method, a series of diffusion couples which was prepared and annealed separately at 1 373 K and 1 473 K for 8 h. The composition-distance profile measured via EPMA was fitted by ERFEX function. The inter-diffusion coefficients and impurity diffusion coefficients were extracted by Whittle-Green method and Hall method. It was found that the diffusion rate of Al in BCC phase of Ti-Al-Cr system is much faster than Cr. Meanwhile main diffusion coefficients and impurity diffusion coefficients of Al increase as the concentration of Al and Cr increase, which is contrary for Cr. The diffusion coefficients measured in experiments were used to assess the atomic mobility and develop the atomic mobility database of the BCC phase in the Ti-Al-Cr system. The comparison between the simulated results (including composition-distance profiles, diffusion paths and main diffusion coefficients) and the experimental data showed a good consistency, which verify the accuracy of the database.
2021 Vol. 26 (3): 189-201 [
Abstract
] (
399
)
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Engineering and Technology
202
Effects of sintering process on microstructures and mechanical properties of Y-Ce-La-TZP ceramics prepared by coating method
WU Fei, LIU Yunzhong, ZHU Kangle
Zirconia powders Co-doped with multiple rare earths were prepared by coating Y(NO
3
)
3
, Ce(NO
3
)
3
and La(NO
3
)
3
on the surface of ZrO
2
by a novel coating method, and then 1.5Y-5.5Ce-0.3La-TZP tetragonal zirconia bulk materials were prepared by pressureless sintering. The effects of sintering temperature,holding time and heating rate on the relative density, apparent porosity, linear shrinkage, phase transformation, mechanical properties and microstructures of zirconia ceramics were studied. The results show that the content of monoclinic phase decreases with the increase of sintering temperature. When the sintering temperature is 1 550 ℃, the content of monoclinic phase is almost zero, and the degree of tetragonal phase stabilization reaches the maximum. At the sintering temperature of 1 200-1 450 ℃, decreasing the heating rate is beneficial to the densification of ceramics. The relative density, bending strength, hardness (HV
10
) and fracture toughness of ZrO
2
ceramics reach the maximum at 1 550 ℃ for 90 min, which are 98.8%, 1 001.3 MPa, 1 124.3 and 10.81 MPa·m
1/2
, respectively.
2021 Vol. 26 (3): 202-210 [
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] (
391
)
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590
)
211
Microstructure and properties of graphite-based composites prepared by warm-pressure curing combined with CVI densification
ZHANG Yang, LI Guodong, HAN Qianwu, JIANG Yi, WANG Yang
Using natural flake graphite powder, mesophase pitch powder and short carbon fiber as raw materials, graphite-based composites are prepared by the method of warm-pressure curing combined with chemical vapor infiltration (chemical vapor infiltration, CVI). The influence of CVI deposited pyrolytic carbon (PyC) on the microstructure and properties of graphite-based composites was studied through X-ray diffraction analysis, scanning electron microscopy and polarized light microscopy, as well as mechanical performance and thermal conductivity testing. The results show that the bending strength of the composites after CVI densification is improved compared with the graphite-based composites doped with short carbon fibers. By introducing PyC into the composite material through CVI, the bending strength of the graphite-based composites increases from 22.5 MPa to 55.9 MPa. Besides, the average grain size of the material and the degree of graphitization increase, which thereby improves the thermal conductivity of the material from 213.24 W/(m·K) to 242.80 W/(m·K).
2021 Vol. 26 (3): 211-218 [
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] (
470
)
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(780 KB) (
832
)
219
Preparation and electrochemical performance of nitrogen-doped carbon-coated porous silicon microspheres
BAI Linghong, BAI Yuting, LEI Ting
Porous silicon (P-Si) microspheres were obtained by dealloying method using AlSi powders as raw material. Then, the surface of P-Si microspheres was coated with polypyrrole (PPy) by in situ chemical oxidation polymerization to get PPy-coated P-Si microspheres (P-Si@PPy). Finally, P-Si@PPy was calcined at 800 ℃ for 3 h in an Ar atmosphere to obtain nitrogen-doped carbon-coated porous silicon (P-Si@NC) microspheres. The morphology, structure, and electrochemical properties of the as-prepared P-Si@NC were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical tests. The results show that the diameter of porous silicon microspheres is about 1 μm and the thickness of nitrogen-doped carbon layer is about 18 nm. When P-Si@NC composites are used as the anode of lithium-ion batteries, the initial discharge specific capacity is up to 2 609.2 mAh/g at a current of 0.1 A/g with a high coulombic efficiency of 87.5%, and a discharge specific capacity of 1 574.8 mAh/g is achieved after 50 cycles, exhibiting excellent electrochemical performance. The porous structure of P-Si@NC provides a buffer space for the expansion/contraction of silicon during the lithiation/delithiation process, and the nitrogen-doped carbon layer not only serves as a protective layer to maintain the particle integrity, but also accelerates the transfer rate of lithium ions and electrons.
2021 Vol. 26 (3): 219-226 [
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] (
408
)
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227
Effect of gradient distribution of grain size on corrosion resistance of 316L stainless steel
CAI Feng, FANG Tiehui
A gradient nanostructure with grain size varied from nanocrystals to coarse grains was prepared on the surface of 316L stainless steel by surface mechanical grinding treatment (SMGT). The corrosion resistance of the gradient sample was measured by chemical immersion experiment and electrochemical corrosion test, and the effect of gradient grain size distribution on corrosion resistance of 316L stainless steel was analyzed. The results show that stable corrosion pits appeared in the nanocrystalline and coarse-grained samples after 20 h immersion, while the surface of the gradient samples remained intact after 50 h immersion. Electrochemical tests show that the corrosion potential of the gradient sample was enhanced from -230 mV (nanocrystalline sample) to -4 mV. Meanwhile, the corrosion current density decreased from 0.137 A/cm
2
to 0.036 A/cm
2
. The grain size gradient distribution does not change the kinetics of passivation film formation. The excellent corrosion resistance of the gradient sample originates from the higher thickness and integrity, lower residual stress of the passivation film.
2021 Vol. 26 (3): 227-234 [
Abstract
] (
388
)
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(644 KB) (
2080
)
235
Influence of N
2
pressure on the microstructure and properties of CrWN coatings
HE Shibin, LU Huihong, TIAN Canxin, LIU Yifei, LI zhujun
The CrWN coatings were prepared by arc ion plating. The effect of nitrogen pressure on the phase structure, morphorlogy, deposition rate, hardness and tribological properties of CrWN coating was investigated. The microstructures, morphology and elements of the CrWN coatings were studied by XRD, SEM and XPS, and hardness tester, friction-wear tester and profilometer were performed to measure their friction coefficient and wear rate. The results show that CrWN coatings is face-centered cubic structure. With the increasing of nitrogen pressure, the diffraction peak intensity of the (111) and (200) of the CrWN coatings has an obvious increase. The numbers and sizes of the large particles in coatings’ surface gradually decrease, and the highest deposition rate is 1.13 μm/h. When nitrogen pressure increases from 0.5 Pa to 1.0 Pa, the microhardness (HV) of CrWN coating increases from 1 900 to 2 010. With further increase of nitrogen pressure, the microhardness of CrWN coating decreases. With the increasing of nitrogen pressure, the average friction coefficient of CrWN coating maintains at 0.5, which is lower than the friction coefficient (0.7) of CrN coating. The wear rate of CrWN coating reaches a maximum of 5.4×10
-7
mm
3
/(N∙m) at nitrogen pressure of 2.0 Pa, which is significantly lower than the wear rate (6.7×10
-7
mm
3
/(N∙m)) of CrN coating.
2021 Vol. 26 (3): 235-242 [
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] (
319
)
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587
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243
Microstructure and mechanical properties of titanium alloys prepared with TiH
2
powder
CHEN Zoujun, LIN Zehua, DUAN Zhongyuan, ZHU Xianzhi, ZHOU Chengshang
Powder metallurgy (PM) Cp-Ti, Ti-6Al-4V and Ti-5Al-2.5Fe alloys were prepared by vacuum sintering process using TiH
2
powder as the raw material. The effects of sintering temperature on microstructure and mechanical tensile properties of sintered products were studied. The results show that the increase of sintering temperature increases the densities and the tensile properties of pure Ti and titanium alloys obviously. After vacuum sintering at 1 200 ℃, the relative density of pure Ti reaches 98.1%, the tensile strength is 501 MPa, and the elongation reaches 11.3%. The relative density of the Ti-6Al-4V alloy is 96.2%, and the tensile strength is 968 MPa, and the elongation is 8.1%. The relative density of the Ti-5Al-2.5Fe alloy is 96.2%, and the tensile strength is 867 MPa, and the elongation is 6.7%. The fracture surface of the three samples sintered at 1 200 ℃ shows a large number of dimples, which exhibits the characteristic of ductile fracture. The strength and elongation of the titanium alloys prepared from TiH
2
powder reach the standard level of titanium alloys.
2021 Vol. 26 (3): 243-249 [
Abstract
] (
375
)
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1022
)
250
Preparation and properties of AlN/Si
3
N
4
/PI composites
WU Yunxiang, LEI Ting, GAO Jiming, YANG Yang
High thermal conductivity polyimide composites have good development prospects in integrated electronic circuits, aerospace and other fields because of their good comprehensive properties. In this paper, silicon nitride whiskers and aluminum nitride particles with high thermal conductivity were selected as inorganic fillers, and surface modification was made with titanate coupling agent, and silicon nitride whiskers/aluminum nitride particles/polyimide (AlN/Si
3
N
4
/PI) composite films were synthesized in situ. The structure and properties of the composite films were characterized by scanning electron microscopy (SEM), Hot Disk tester and tensile tester. The results show that when the volume ratio of AlN/Si
3
N
4
is 4:2, the composite filler forms a good thermal conductivity network in the matrix. When the total filler volume fraction reaches 30%, the thermal conductivity of the composite film reaches the maximum of 0.84 W/(m·K). The mechanical properties of the composites decrease with the increase of filler content, and the heat resistance increases with the increase of filler content.
2021 Vol. 26 (3): 250-256 [
Abstract
] (
455
)
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1016
)
257
Effects of ZrN doping on magnetic properties and electrical resistivity of NdFeB magnets prepared by hot pressing and hot deformation
BAI Yang, WANG Ziliang, LI Yajing, ZUO Siyuan, FANG Yikun, ZHU Minggang, HUANG Guangwei, ZHENG Liyun
NdFeB magnets with different mass fractions of ZrN were prepared by mixing ZrN and NdFeB melt quenching powder evenly, followed by hot pressing and hot deformation. The effects of ZrN doping on the hot pressing density, magnetic properties, uniformity and electrical resistivity of the hot pressed and hot deformed NdFeB magnets were studied. The results show that the magnetic properties in the interior of the hot pressed magnet are the best, which gradually deteriorate along the radial direction. With the increase of the mass fraction of ZrN, the density of the hot pressed magnets increases first and then decreases. The electrical resistivity of the hot deformed magnets gradually increases with the mass fraction of ZrN, while the magnetic properties decrease linearly. When the mass fraction of ZrN is 1%, the remanence magnetization, the coercivity, the maximum magnetic energy product are 1.362 T, 865.5 kA/m and 349.6 kJ/m
3
, respectively. And the resistivity is 250 μΩ∙cm. Compared with that of the undoped hot deformed NdFeB magnet (230 μΩ∙cm), the resistivity of the doped NdFeB magnet increases by about 8.7%.
2021 Vol. 26 (3): 257-262 [
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] (
387
)
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(438 KB) (
993
)
263
Effect of Ni content on the solid solution strengthening behavior of Cu-Ni-Ag alloys
WANG Jingying, LÜ Xinqun, CHEN Shiqi, GAO Yang, LIU Yong
In this paper, different Ni elements were added in the Cu-Ni-Ag alloy, and gas atomized alloy powders were used to manufacture bulk sample by hot pressing, rolling and annealing. The results indicated that the Cu-Ni-Ag alloy with high strength and good plasticity has been successfully fabricated by composition and microstructure optimization. The main strengthening mechanisms of Cu-Ni-Ag alloy are solution strengthening and dislocation strengthening. The solution strengthening effect of Ni is significant, but the solution solubility of Ag in the alloy is reduced by Ni. The number of Ag-rich precipitates increases with the increase of Ni content. The precipitated phase can further enhance the strength by pinning grain boundaries and impeding dislocation movement.
2021 Vol. 26 (3): 263-271 [
Abstract
] (
433
)
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1331
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272
Effect of alkali-heat treatment on the surface microstructure of Ti-Ta composite materials
CHEN Manke, LIU Yong, XU Shenghang, HUANG Qianli
Ti-Ta composite material has high specific strength, low modulus and good biocompatibility, and has potential applications in the fields of biomedicine and chemical industry. In this paper, a highly dense Ti-Ta composite material with a nominal composition of Ti-20%Ta (mole fraction) was prepared by spark plasma sintering and thermal deformation using Ti and Ta powders as raw materials. The microstructure of the material was analyzed and the surface state of the material was modified by surface sandblasting and alkali heat treatment. The results show that Ta particles are uniformly distributed in the Ti matrix, and the interface between Ta and Ti has an apparent compositional gradient, but the diffusional range is narrow. Compared to Ti-Ta composite material surfaces treated by sand blasting, those treated by sand blasting and alkali-heat treat mentexhibits enhanced cyto-compatibility. The reason for the improvement of biological properties of modified Ti-Ta composite material may be that the nano-pore structure formed on the surface is helpful for protein adsorption, and the sodium titanate and sodium tantalate formed on the surface have better adsorption properties for protein. Therefore, surface alkali-heat treatment is an effective way to improve the biocompatibility of Ti-Ta composite material.
2021 Vol. 26 (3): 272-277 [
Abstract
] (
436
)
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839
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278
Preparation and mechanical property of C
f
/C-ZrB
2
composite material by slurry brushing and needle punching
SHI Qi, LI Guodong, WANG Yang, JIANG Yi
In this paper, C
f
/C-ZrB
2
composite material was prepared by natural graphite powder, ZrB
2
powder, alcohol-soluble phenolic resin powder, carbon fiber felt, carbon fiber latitude cloth, which involved the processes including slurry brushing, needle punching, temperature and pressure curing, high temperature carbonization process, as well as chemical vapor deposition. Density and porosity were tested by Archimedes drainage method, the impacts of slurry solid content on the distribution of matrix powder in the C
f
/C-ZrB
2
porous body and mechanical properties of the composite material were tested by using scanning electron microscopy analysis and mechanical performance testing and other methods. The results show that increasing the solid content of the slurry improves the uniformity of the matrix powder distribution in the green body. The mechanical properties of the material increase first and then decrease with the increase of the solid content. When the volume fraction of the solid phase of the slurry is 15%, the bending strength reaches 121.86 MPa.
2021 Vol. 26 (3): 278-284 [
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] (
303
)
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872
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